The Hall of Fame said in a statement that Seaver passed peacefully in his sleep of complications of Lewy body dementia and COVID-19.
"We are heartbroken to share that our beloved husband and father has passed away," said his wife Nancy Seaver and daughters Sarah and Anne in a statement. "We send our love out to his fans, as we mourn his loss with you."
Also read: New Robin Williams film examines Lewy body dementia
The Mets released a statement on Twitter:
Seaver transformed the franchise and captivated the city, setting enduring standards as he whipped his powerful right arm overhead for the Miracle Mets and dirtied his right knee atop major league mounds for two decades.
Related: The 15 greatest moments in Mets postseason history
A consummate pro and pitching icon, he finished fulfilled after a career remembered with awe long after his final strikeout.
"It is the last beautiful flower in the perfect bouquet," Seaver said on the afternoon he was inducted into baseball's Hall of Fame.
Seaver was the galvanizing force who steered the Mets from National League laughingstock to a stunning World Series title in 1969, and he was so good he was given two nicknames: "Tom Terrific" and "The Franchise."
"He will always be the heart and soul of the Mets, the standard which all Mets aspire to," Mike Piazza, a former Mets catcher and Hall of Famer, tweeted when Seaver's dementia diagnosis was announced.
Tom Seaver was a larger than life baseball Icon, he embodied all that is desired in a player and a man, dedicated, loyal, sincere and fiercely competitive. Showed strength and dignity during his recent suffering. He will be missed #Mets #TomSeaver— Mike Piazza (@mikepiazza31) September 3, 2020
He was selected by the Mets in a special draft in 1966 after his contract with the Braves was nullified by Major League Baseball. He spent just one year in the minors and won the 1967 National League Rookie of the Year with a 16-13 record and 2.76 ERA for the last-place Mets.
Said ex-Mets closer and captain John Franco: "As a kid, you always wanted to be Tom Seaver."
Seaver won the Cy Young three times, all with the Mets. He remains the franchise leader in most career categories, including victories with 198 victories ad strikeouts with 2,541.
"Tom was a gentleman who represented the best of our national pastime," Commissioner Rob Manfred said in a statement. "He was synonymous with the New York Mets and their unforgettable 1969 season...After their improbable World Series championship, Tom became a household name to baseball fans - a responsibility he carried out with distinction throughout his life," he said.
Seaver's plaque in Cooperstown lauds him as a "power pitcher who helped change the New York Mets from lovable losers into formidable foes."
Sad news for sure. My idol growing up was Tom Seaver. I am forever grateful and blessed to have been able to befriend this great man. Tom Terrific was a classy gentleman and the greatest @Mets May he Rest In Peace #franchise #mets #HOF pic.twitter.com/KEfmUy7I2d— Al Leiter (@AlLeiter22) September 3, 2020
He was the first pitcher in major league history with eight consecutive seasons of at least 200 strikeouts, and he was a five-time 20-game winner in 1969, 1971, 1972, 1975, and 1977.
Seaver finished his Hall of Fame career with a 311-205 record, a 2.86 ERA, 3,640 strikeouts, 61 shutouts and 231 complete games.
"My biggest disappointment? Leaving the Mets the first time and the difficulties I had with the same people that led up to it," Seaver told The Associated Press ahead of his Hall induction in 1992. "But I look back at it in a positive way now. It gave me the opportunity to work in different areas of the country."
He threw his only no-hitter for the Reds in June 1978 against St. Louis and was traded back to New York after the 1982 season. But Mets general manager Frank Cashen blundered by leaving Seaver off his list of 26 protected players, and in January 1984 he was claimed by the Chicago White Sox as free agent compensation for losing pitcher Dennis Lamp to Toronto.
While pitching for the White Sox, Seaver got his 300th win at Yankee Stadium and did it in style with a six-hitter in a 4-1 victory. He finished his career with the 1986 Boston Red Sox team that lost to the Mets in the World Series.
"Just a class act. Just a gentleman in the way he handled himself, and really the way he handled his whole career," said Miami manager Don Mattingly, a former New York Yankees captain. "We just left New York, and every time you walk in a door there, it's like Tom Seaver Hall, with different pictures."
When the Mets closed their previous home, Shea Stadium, on the final day of the 2008 regular season, Seaver put the finishing touches on the nostalgic ceremonies with a last pitch to Piazza, and the two walked off together waving goodbye to fans.
"He was a star. A shining star," Mets teammate Ed Kranepool said. "No one can replace Tom Seaver."
Seaver is survived by Nancy, daughters Sarah and Anne, and grandsons Thomas, William, Henry and Tobin.
Information from the Associated Press was used in this report.
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